a) Field of the invention
The present invention relates to an imaging optical system for endoscopes, and more specifically to an imaging optical system suited for use with imaging systems which can be connected to image guide fiber bundles used for fiber catheters.
b) Description of the prior art
FIG. 1 shows a configuration of an imaging system which is used for observing interiors of body cavities, machines, etc. on a monitor TV. In this drawing, the reference numeral 1 represents a fiber scope which accommodates an observation optical system consisting of an objective lens 2, an image guide fiber bundle 3, an eyepiece lens 4, etc., and an illumination system consisting of a light source 5, a light guide fiber bundle 6, etc., and is adapted to permit observation of an object O through the observation optical system while illuminating the object with the illumination system. When an image of the object formed by the observation optical system and projected onto a monitor TV is to be observed, an adapter lens 7 is arranged after the eyepiece lens 4 and a TV camera 9 having a built-in image pickup device is arranged thereafter so that an image formed on the end surface of emergence of the image guide fiber bundle is projected onto the image pickup device 8 through the eyepiece lens 4 and the adapter lens 7. Output signals provided from the image pickup device 8 are fed into a camera control unit 10 comprising a signal processing circuit for performing predetermined signal processing and then inputting the signals into a monitor TV 11.
The imaging system of the above-described type, in which an image of an object formed by a fiber scope is projected onto a monitor TV by using the adapter lens, has a defect that it allows size of the image to be variable on the monitor TV depending on the types of fiber scopes employed. The fiber scopes are equipped with image guide fiber bundles different in thickness thereof and eyepiece lenses having different magnifications.
In order to correct this defect, it is desired in these days to use a zoom lens system as the adapter lens. For example, there is known the imaging system disclosed by Japanese Patent Kokai Publication No. Sho 62-285454 which uses, as an adapter lens, a zoom lens system capable of providing optimum magnifications and image brightness even when combined with various types of fiber scopes, non-flexible endoscopes and so on.
FIG. 2 shows a sectional view illustrating a conventional example a of zoom lens system to be used as an adapter lens. This zoom lens system consists, in the order from the side of the eyepiece lens, a first lens unit L.sub.1 functioning as a compensator, a second lens unit L.sub.2 functioning as a variator and a third lens unit having an imaging function.
Further, as another example of zoom lens system which is used as an eyepiece lens for endoscopes, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 58-66911 which consists of a positive lens unit and a negative lens unit.
In the field of endoscopes, attention is now concentrated on the fiber scope which is designed for observation of blood vessels. This fiber scope is very thin, or equipped with a distal end having an outside diameter of approximately 1 mm, and is used for observing not only blood vessels but also pancreatic ducts and so on.
The image guide fiber bundle which is designed for use with such a fiber scope has a very small outside diameter on the order of several-tenths of a millimeter. Accordingly, the end surface of the image guide fiber bundle narrows the apparent visual field and makes observation rather inconvenient when observation is through an eyepiece lens in he conventional manner. Further, also when an adapter lens is attached to the eyepiece lens for imaging, the image guide fiber bundle poses a problem that the conventional zoom type adapter lens system has too low a magnification for satisfactory observation.